This invention relates generally to methods and devices which test the ability of equipment and especially electronic equipment, to survive a nuclear detonation. More specifically, this invention relates to a method and device which simulates, without a nuclear detonation, certain aspects of the environment produced by such a detonation. The environment in question is the source-region environment, i.e., the area close enough to an atmospheric burst that ionization of the air is an essential component. The environment is characterized by blast effects, thermal radiation, neutron radiation, gamma radiation, intense pulsed electromagnetic fields (EMP) and time varying (pulsed) air conductivity. The present invention simulates the aspects of gamma radiation, intense pulsed electromagnetic fields (EMP) and time varying (pulsed) air conductivity. In the vicinity of an atmospheric nuclear detonation, the EMP and air conductivity are caused by the gamma flux. Compton electrons, set into motion by the gamma rays, act as the sources of both local and radiated fields, and, in slowing down, lose energy through the ionization of air molecules. This ionization makes the air conductive.
There are several source region electromagnetic pulse (SREMP) simulators one of which is the AURORA Flash X-Ray Facility at the U.S. Army Electronics Research and Development Command, HARRY DIAMOND LABORATORIES located in Adelphi, Maryland. The AURORA is considered as a self-contained SREMP simulator but suffers from several severe limitations. In the AURORA, bremsstrahlung is produced in four thick tantalum targets by four synchronous 10-MeV electron beams. The bremsstrahlung then produces an EMP signal through the same mechanism as does a nuclear detonation. It induces Compton electron currents in the AURORA test chamber. As the Compton electrons are slowed down, they ionize the air and also produce an electromagnetic field in the chamber. However, the EMP produced in the AURORA is not a true model of the EMP produced by a nuclear detonation.
In both AURORA and a true SREMP environment, the early-time electromagnetic phenomena can be roughly considered as evolving in two successive phases:
a. the wave phase (when D.quadrature.&gt;&gt;.sigma.E, during the fast turn-on of the Compton drivers); and
b. the diffusion phase (when .sigma.E&gt;&gt;D). During the wave phase, the spherical domain of influence (the volume, surrounding a field point, containing sources determining the fields at the field point) has radius C(t-t.sub.0), where C is the speed of light and the pulse begins at time t.sub.0. During the diffusion phase, the radius of influence contracts as .sigma. increases, and when .sigma. levels off, the radius increases as .sqroot.t rather than linearly.
The limitations of the AURORA test facility as a nuclear EMP similator can be understood by examining its response in terms of the wave equation for the E and H fields. To simplify the examination it is assumed that the examination point is far enough away from the AURORA test cell "hot spot" (the source of radiation) so that a spatially homogeneous conductivity can be assumed. The two wave equations which express the electric and magnetic fields in terms of the Compton current J and charge density .rho.: ##EQU1## using conventional notation. If it is assumed that the EMP that is to be simulated is in the diffusion phase, then ##EQU2## Another assumption that can be made ##EQU3## though not strictly valid leads to great simplification of the wave equations. However, a more thorough treatment involving the renormalization of time does justify the qualitative discussion presented. With these assumptions, the E and H fields inside the AURORA test cell are ##EQU4## Now, considering the diffusion Green function given by ##EQU5## If a value of .sigma.=3.times.10.sup.-4 mho/m, 10-m from the hot spot, and a time equal to 10.sup.-7 seconds (roughly the FWHM of the AURORA pulse) is substituted in the Green function then ##EQU6## for the exponential factor. In other words, Compton drivers and charges within a spherical gaussian "domain of influence" of standard deviation 30 meters contribute to the local E and H fields in a tactical situation. The spatial distribution of source currents in the AURORA test cell has much too small an extension to generate a reasonable tactical EMP simulation. Another limitation of the AURORA test cell is that the metallic walls of the cell short-circuit the E field, an effect seen throughout the cell because of the relatively large skin depth of the ionized joint. A third limitation is that the radiation pulses rise-time (and hence the rise-times of the fields and conductivity) is too long. Because of these limitations of existing radiation sources a need for an auxiliary source of pulsed fields is needed. One approach has been to place a guided-wave structure in the test cell to provide a traveling electromagnetic pulsed wave. A large (12 m.times.4 m.times.3 m) transmission line has been mounted laterally in the AURORA test cell, see FIGS. 1 & 2 which represents prior art efforts in the AURORA. The line is driven by a 100 KV pulser which provides the appropriate propagating electric and magnetic fields. Simultaneously, AURORA is fired to provide a time-varying pulse of ionizing radiation. However, these two effects are not independent, resulting in a failure of the system to provide a true modeling of an actual nuclear detonation. The time-varying conductivity creates a varying load on the pulser-line system and thus distorts the voltage pulse, and consequently, the electric field inside the line (the test area). The fast-rising load current increases the magnetic field in the line, resulting in a negative voltage pulse and a falling E-field; and subsequently, as the load impedance rises again, the magnetic field energy continues to drive current into it, resulting in a positive voltage pulse and a rising and overshooting E-field. This overshooting effect is referred to as the inductive kick.
It is therefore one object of this invention to provide a method to simulate the electromagnetic pulse (EMP) created in the near vicinity of a nuclear detonation.
It is another object of this invention to provide a method to simulate the electromagnetic pulse (EMP) created in the near vicinity of a nuclear detonation that can be utilized in conjunction with a source of time-varying pulses of ionizing radiation.
It is a further object of this invention to provide an apparatus to simulate the electromagnetic pulse (EMP) created in the near vicinity of a nuclear detonation.
It is still a further object of the invention to provide an apparatus to simulate the electromagnetic pulse (EMP) created in the near vicinity of a nuclear detonation that can be utilized in conjunction with a source of time-varying pulses of ionizing radiation.